An internal combustion engine of the stated type is used as a motor vehicle drive unit. Within the context of the present disclosure, the expression “internal combustion engine” encompasses diesel engines and Otto-cycle engines and also hybrid internal combustion engines, which utilize a hybrid combustion process, and hybrid drives which comprise not only the internal combustion engine but also an electric machine which can be connected in terms of drive to the internal combustion engine and which receives power from the internal combustion engine or which, as a switchable auxiliary drive, additionally outputs power.
Internal combustion engines have a cylinder block and at least one cylinder head which are connected to one another to form the cylinders, that is to say the combustion chambers. The cylinder block, as the upper crankcase half, generally serves for the mounting of the crankshaft and for accommodating the piston and the cylinder liner of each cylinder. The cylinder head normally serves for accommodating the valve drives required for the charge exchange.
During the course of the charge exchange, the discharge of the combustion gases via the exhaust-gas discharge system takes place via the at least one outlet opening, and the feed of the combustion air via the intake system takes place via the at least one inlet opening of the cylinder. In the case of four-stroke engines, use is made almost exclusively of lifting valves for the control of the charge exchange. The actuating mechanism, including the associated valve, is referred to as valve drive.
The crankshaft which is mounted in the crankcase absorbs the connecting rod forces and transforms the oscillating stroke movement of the pistons into a rotational movement of the crankshaft. The upper crankcase half formed by the cylinder block is generally supplemented by the oil pan, which can be mounted on the cylinder block and which serves as the lower crankcase half. The oil pan serves to collect and store the engine oil and is commonly part of the oil circuit. To hold and mount the crankshaft, at least two bearings are provided in the crankcase.
In the development of internal combustion engines, it is constantly sought to minimize fuel consumption and reduce pollutant emissions.
Fuel consumption poses a problem in particular in the case of Otto-cycle engines, that is to say in the case of spark-ignition internal combustion engines. The reason for this lies in the principle of the operating process of the traditional Otto-cycle engine. The traditional Otto-cycle engine operates with external mixture formation and a homogeneous fuel-air mixture, in which the desired power is set by varying the charge of the combustion chamber, that is to say by means of quantity regulation. By adjusting a throttle flap which is provided in the intake system, the pressure of the inducted air downstream of the throttle flap can be reduced to a greater or lesser extent. For a constant combustion chamber volume, it is possible in this way for the air mass, that is to say the quantity, to be set by means of the pressure of the inducted air. This also explains why quantity regulation has proven to be disadvantageous specifically in part-load operation, because low loads demand a high degree of throttling and a pressure reduction in the intake system, as a result of which the charge exchange losses increase with decreasing load and increasing throttling.
One approach for dethrottling the Otto-cycle working process is to utilize direct fuel injection. The injection of fuel directly into the combustion chamber of the cylinder is considered to be a suitable measure for noticeably reducing fuel consumption even in Otto-cycle engines. The dethrottling of the internal combustion engine is realized by virtue of quality regulation being used within certain limits.
With the direct injection of the fuel into the combustion chamber, it is possible in particular to realize a stratified combustion chamber charge, which can contribute significantly to the dethrottling of the Otto-cycle working process because the internal combustion engine can be leaned to a great extent by means of the stratified charge operation, which offers thermodynamic advantages in particular in part-load operation, that is to say in the lower and middle load range, when only small amounts of fuel are to be injected. The stratified charge is characterized by a highly inhomogeneous combustion chamber charge, wherein an ignitable fuel-air mixture with relatively high fuel concentration is present in the region of the ignition device.
There is relatively little time available for the injection of the fuel, for the mixture preparation in the combustion chamber, specifically the mixing of air and fuel and the preparation of the fuel within the context of preliminary reactions including evaporation, and for the ignition of the prepared mixture.
The resulting demands placed on the mixture formation relate not only to the direct-injection Otto-cycle engine but basically to any direct-injection internal combustion engine, and thus also to direct-injection diesel engines. The internal combustion engine to which the present disclosure relates is very generally a direct-injection internal combustion engine.
Since, in the case of direct injection, there is only little time available for the mixture formation, there is a demand, inter alia, for a combustion chamber geometry by way of which the mixture formation is assisted and accelerated in order to substantially homogenize the fuel-air mixture before the ignition, at least as long as there is no demand for stratified-charge operation. The piston takes on a particular significance in this context, wherein the combustion chamber is formed jointly by the piston crown of said piston together with the cylinder liner and the cylinder head.
The mixture formation is substantially assisted and accelerated in that a forced charge movement in the combustion chamber, for example a tumble or a swirl, ensures good mixing of the intake air with the injected fuel. The fuel may also be injected into the combustion chamber in such a way that the injection jet is targeted toward a wall delimiting the combustion chamber, preferably into a depression provided on the piston crown. It is the intention for the fuel jet to be, as a result of the impingement, broken up into multiple jet parts and diverted such that as large an area of the combustion chamber as possible is encompassed by the fuel jets. The transportation and the distribution of the fuel are furthermore assisted by the impetus of the injection jet.
In direct-injection internal combustion engines which are operated with an excess of air, that is to say for example direct-injection diesel engines but also direct-injection Otto-cycle engines, the nitrogen oxides contained in the exhaust gas cannot be reduced out of principle, that is to say on account of the lack of reducing agents, for example, carbon monoxide or unburned hydrocarbons, in the exhaust gas. Furthermore, owing to the more or less inhomogeneous fuel-air mixture, soot emissions represent a problem.
For the reduction of the nitrogen oxide emissions of an internal combustion engine, a distinction can be made between two fundamentally different approaches.
In a first approach, it is sought to influence the combustion process such that the fewest possible nitrogen oxides arise, that is to say are formed, during the combustion of the fuel in the first place.
Since the formation of the nitrogen oxides requires not only an excess of air but also high temperatures, combustion processes with relatively low combustion temperatures, so-called LTC methods, are, inter alia, expedient for the reduction of the untreated emissions of nitrogen oxides.
Low combustion temperatures may be realized by virtue of the ignition retardation being increased, and the rate of combustion being reduced. Both can be achieved through the admixing of combustion gases to the cylinder fresh charge or by increasing the exhaust-gas fraction in the cylinder fresh charge, whereby exhaust-gas recirculation is to be regarded as a suitable measure for lowering the combustion temperature, specifically both external exhaust-gas recirculation, that is to say the recirculation of combustion gases from the exhaust-gas side to the intake side of the internal combustion engine, and internal exhaust-gas recirculation, that is to say the retention of exhaust gases in the cylinder during the charge exchange. With increasing exhaust-gas recirculation rate, the nitrogen oxide emissions can be considerably reduced.
To obtain an adequate or noticeable reduction in nitrogen oxide emissions, high exhaust-gas recirculation rates are required which may be of the order of magnitude of xEGR≈60% to 70%. Therefore, the hot exhaust gas is preferably cooled during the course of the recirculation. The cooling of the recirculated exhaust gas facilitates, or permits, the realization of high recirculation rates. The lowering of the temperature of the exhaust gas during the course of the cooling leads to an increase in density, and to a smaller exhaust-gas volume for a given exhaust-gas mass. Furthermore, the cooling of the recirculated exhaust gas assists the lowering of the combustion temperature, because this also results in the temperature of the entire cylinder fresh charge being lowered.
Not only the untreated nitrogen oxide emissions but also the untreated soot emissions are reduced as a result of the measures described above.
Even though the low-temperature method, that is to say the LTC method, yields considerable improvements with regard to pollutant emissions, said approach is subject to limits. Specifically, at high loads, it is generally not possible to realize high recirculation rates, wherein it is specifically at high loads that the highest process temperatures arise, which would have to be lowered in order to reduce the untreated nitrogen oxide emissions.
The second approach for reducing the nitrogen oxide emissions comprises in aftertreatment of the exhaust gas that is formed during the combustion, and of the pollutants contained therein. To reduce pollutant emissions, internal combustion engines may be equipped with various exhaust-gas aftertreatment systems.
The use of exhaust-gas aftertreatment systems is likewise associated with disadvantages. Firstly, the aftertreatment of exhaust gases is expensive, in particular owing to the required coating of the catalytic converters with high-grade metals. Secondly, it may be taken into consideration that different aftertreatment systems are used for the various pollutants, that is to say a multiplicity of systems is required, and the systems have a limited service life and may even have to be replaced prematurely in the event of damage. The installation of the exhaust-gas aftertreatment systems may also lead to packaging problems.
Furthermore, the operation of the exhaust-gas aftertreatment systems is associated with disadvantages, for example the use of fuel for regenerating or maintaining the functionality of the exhaust-gas aftertreatment systems, and associated oil thinning. The exhaust-gas aftertreatment may furthermore limit the operation of the internal combustion engine.
The above statements make it clear that it is basically advantageous to keep the untreated emissions as low as possible in order to thereby minimize the outlay in the context of an exhaust-gas aftertreatment system. In this respect, aside from the LTC method and aside from exhaust-gas recirculation, further measures are desired to reduce the untreated emissions of a direct-injection internal combustion engine.
Against the background of that stated above, it is an object of the present disclosure to provide a direct-injection internal combustion engine as per the preamble of claim 1, in the case of which the fuel-air mixture situated in the combustion chamber is homogenized more effectively before the ignition, and the untreated emissions are lower.
The inventors herein have recognized the above issues and identified an approach to at least partly address the issues. In one example approach, a direct-injection internal combustion engine having at least one cylinder head comprising at least one cylinder and having a crankshaft rotatably mounted in a crankcase, where each cylinder comprises a combustion chamber which is jointly formed by a piston crown of a piston associated with the cylinder, by a cylinder liner and by the at least one cylinder head, and which internal combustion engine is characterized in that the piston is equipped at least regionally with a surface structure, wherein more than 50% of the piston crown of the piston is equipped with a surface structure. In this way, the surface structure may beneficially alter combustion conditions.
The piston of the internal combustion engine according to the present disclosure has a structure at least regionally on its surface, that is to say on the outer side which delimits the piston at the outside. This has numerous positive effects.
Firstly, the structure increases the surface area of the piston and thus also the heat-transmitting surface area between the combustion chamber and the piston, or between the piston and the surroundings.
The heat released in the combustion chamber during the combustion by the exothermic, chemical conversion of the fuel is dissipated partially via those walls of the cylinder head, of the cylinder block and of the piston which delimit the combustion chamber and partially via the exhaust-gas flow. The equipping of the piston with a surface structure, and the associated increase in size of the heat-transmitting surface area, leads to an increase in the amount of heat that is dissipated from the combustion chamber via the piston, and thus to a lowering of the cylinder interior temperature and of the process temperatures. Here, both that side of the piston which faces toward the combustion chamber, and that side of the piston which faces toward the crankcase, that is to say the piston bottom side, are of relevance.
As already discussed in conjunction with the LTC method or the exhaust-gas recirculation, lower process temperatures lead to lower untreated nitrogen oxide and soot emissions.
The concept according to the present disclosure has advantages in particular at high loads, in the case of which it is generally not possible to realize high recirculation rates. This is because the enlarged heat-transmitting surface area of the piston is effective even at high loads, and ensures, together with the then high process temperatures, an increased dissipation of heat from the combustion chamber, that is to say a particularly pronounced lowering of the process temperature and thus reduction of the untreated emissions.
Secondly, the surface structure of the piston gives rise to additional micro-turbulence close to the surface of the piston, and thus a more intense charge movement in the combustion chamber. The surface structure assists and accelerates the mixture formation, that is to say the mixing of the air situated in the cylinder with the injected fuel. In particular, wetting of the piston crown with liquid fuel is counteracted, whereby the emissions of unburned hydrocarbons can be reduced.
The homogenization of the fuel-air mixture is promoted by the additional charge movement in the combustion chamber, whereby improved utilization of the energy bound in the fuel is also realized, that is to say an improvement in efficiency can be achieved.
With the internal combustion engine according to the present disclosure, the first object on which the present disclosure is based is achieved, that is to say an internal combustion engine is provided in the case of which the fuel-air mixture situated in the combustion chamber is homogenized more effectively before the ignition, and the untreated emissions are lower.
According to the present disclosure, more than 50% of the piston crown of the piston is equipped with a surface structure.
Embodiments of the internal combustion engine are also advantageous in which more than 70% of the piston crown of the piston is equipped with a surface structure.
Embodiments of the internal combustion engine are likewise advantageous in which more than 80% of the piston crown of the piston is equipped with a surface structure.
Embodiments of the internal combustion engine may also be advantageous in which the entire piston crown of the piston is equipped with a surface structure.
Embodiments of the internal combustion engine are advantageous in which the crankshaft is articulatedly connected to the piston of each cylinder such that, as the crankshaft rotates about an axis of rotation, the piston oscillates along a piston longitudinal axis, the piston longitudinal axis being perpendicular to the axis of rotation.
Embodiments of the internal combustion engine are advantageous in which each cylinder is equipped with an injection device which is arranged in the cylinder head, on the side facing the piston crown, for the purposes of directly injecting fuel into the combustion chamber of the cylinder.
Embodiments of the internal combustion engine are advantageous in which each cylinder has at least one inlet opening for the supply of the combustion air via an intake system and at least one outlet opening for the discharge of the combustion gases via an exhaust-gas discharge system.
It is the object of the valve drive to open and close the inlet and outlet openings of the cylinder at the correct times, with a fast opening of the greatest possible flow cross sections being sought in order to keep the throttling losses in the inflowing and outflowing gas flows low and in order to ensure increased charging of the combustion chamber with combustion air, and an effective discharge of the exhaust gases. Cylinders are therefore preferably equipped with two or more inlet openings and outlet openings respectively.
Embodiments of the internal combustion engine are advantageous in which the piston crown of each piston has a depression which comprises a depression base and walls which circumferentially laterally delimit the depression base.
The geometry of the piston crown, in particular a piston depression provided in the piston crown, has a significant influence on the charge movement and thus on the mixture formation in the combustion chamber. In the case of direct-injection internal combustion engines, use is generally made of depressions that are rotationally symmetrical with respect to the piston longitudinal axis, in particular omega-shaped depressions.
In this connection, embodiments of the internal combustion engine are advantageous in which the depression of the piston is equipped with a surface structure. This is advantageous in particular in embodiments in which the at least one injection jet is directed into the depression.
Embodiments of the internal combustion engine are advantageous in which the piston has multiple protruding elements for forming the surface structure.
In this context, embodiments of the internal combustion engine are advantageous in which the piston has multiple convex elements, that is to say outwardly domed elements, for forming the surface structure. Use may be made of spherical elements or conical or frustoconical elements. The cross section of the elements may be round, circular or elliptical, angular, polygonal or the like, and may also vary.
Here, embodiments of the internal combustion engine are advantageous in which the piston has multiple stud-like elements for forming a cauliflower-like surface structure. The elements may also transition into one another and form Siamese overlaps, that is to say may share regions of the piston.
In this connection, embodiments of the internal combustion engine are also advantageous in which the piston has multiple rib-like elements for forming the surface structure.
Embodiments of the internal combustion engine are also advantageous in which the piston has multiple recesses for forming the surface structure. The recesses are the counterpart, that is to say of the opposite design, to the protruding elements, specifically in the form of openings or hollows.
In this context, embodiments of the internal combustion engine are advantageous in which the piston has multiple concave recesses, that is to say inwardly domed recesses, for forming the surface structure.
The recesses, like the protruding elements, may also transition into one another and share regions of the piston. The recesses or elements provided for forming the surface structure may however also be arranged spaced apart from one another.
Embodiments of the internal combustion engine are advantageous in which the surface structure has a height of less than 5 millimeters, wherein the height refers to the spatial extent of the structure perpendicular to the surface of the piston.
Embodiments of the internal combustion engine are advantageous in which the surface structure has a height of less than 3 millimeters, wherein the height refers to the spatial extent of the structure perpendicular to the surface of the piston.
Embodiments of the internal combustion engine are advantageous in which the piston is, on a side facing toward the combustion chamber, equipped at least regionally with a surface structure.
In the present case, both effects imparted by the surface structure come to bear. Firstly, the heat-transmitting surface area between the combustion chamber and the piston is enlarged, and thus the heat transfer or the heat dissipation from the combustion chamber via the piston is increased. Secondly, the surface structure of the piston gives rise to an intensified charge movement in the combustion chamber.
Embodiments of the internal combustion engine are also advantageous in which the piston is, on a side facing toward the crankcase, equipped at least regionally with a surface structure. The enlargement of the heat-transmitting surface area on the underside of the piston increases the heat transfer between the piston and surroundings, in particular if an oil spray-type cooling arrangement is used to cool the underside of the piston.
Embodiments of the direct-injection internal combustion engine are advantageous in which a supercharging arrangement, preferably an exhaust-gas turbocharging arrangement, is provided.
Here, for supercharging, use is preferably made of at least one exhaust-gas turbocharger in which a compressor and a turbine are arranged on the same shaft, with the hot exhaust-gas flow being supplied to the turbine and expanding in said turbine with a release of energy, whereby the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor conveys and compresses the charge air fed to it, as a result of which supercharging of the cylinders is obtained.
The advantages of the exhaust-gas turbocharger for example in relation to a mechanical charger are that no mechanical connection for transmitting power exists or is required between the charger and internal combustion engine. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
Nevertheless, embodiments of the internal combustion engine may also be advantageous in which the supercharging is realized by means of at least one mechanical charger, for example a compressor, if appropriate also in combination with an exhaust-gas turbocharging arrangement.
Supercharging serves primarily to increase the power of the internal combustion engine. Here, the air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
The greater amount of fuel injected per working cycle in the case of a supercharged internal combustion engine places particularly high demands on the mixture formation in order to ensure adequate mixing of the air situated in the cylinder with the injected fuel, that is to say in order to adequately homogenize the fuel-air mixture.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and a more expedient power-to-weight ratio. If the swept volume is reduced, it is thus possible, given the same vehicle boundary conditions, to shift the load collective toward higher loads, at which the specific fuel consumption is lower. Supercharging of an internal combustion engine consequently assists in the efforts to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine.
With targeted configuration of the supercharging, it is also possible to obtain advantages with regard to exhaust-gas emissions. With suitable supercharging for example of a diesel engine, the nitrogen oxide emissions can therefore be reduced without any losses in efficiency. The hydrocarbon emissions can be favorably influenced at the same time. The emissions of carbon dioxide, which correlate directly with fuel consumption, likewise decrease with falling fuel consumption.
The second sub-object on which the present disclosure is based, specifically that of specifying a method for producing a piston of a direct-injection internal combustion engine of a type described above, is achieved by way of a method which is distinguished by the fact that the piston is equipped at least regionally with a surface structure.
That which has already been stated with regard to the internal combustion engine according to the present disclosure also applies to the method according to the present disclosure.
Embodiments of the method are advantageous in which the piston is produced as a blank in a casting process, and finish machining of the blank is performed, during the course of which the surface structure is formed. The casting of the piston is particularly suitable for series production.
Here, embodiments of the method are shown in which the surface structure is formed by way of a coating process.
Embodiments of the method are also shown where the surface structure is formed together with the piston blank during the casting process.